1. Technical Field
A method for fabricating a CMOS image sensor is disclosed and, more particularly a method for fabricating a CMOS image sensor is disclosed that reduces the amount of dark current by protecting the surface of a photodiode with spacer etching barrier film during the etching process for formulation of spacers.
2. Description of the Related Art
In general, an image sensor is a semiconductor device that converts an optical image into electrical signals. In a charge coupled device (CCD), a plurality of Metal-Oxide-Silicon (MOS) capacitors is placed in close proximity, and charge carriers are stored in or transferred between capacitors. CMOS image sensors are devices using as many MOS transistors as the number of pixels to detect output sequentially, and is based on CMOS technology that uses peripheral circuits such as control circuits and signal processing circuits.
As is well known, an image sensor for color images has a color filter array (CFA) on top of a photo sensing part, which generates and stores photo-generated charges in response to external lights. The color filter array (CFA) has 3 colors of Red, Green and Blue, or 3 colors of Yellow, Magenta and Cyan.
Also, the image sensor consists of a photo sensing part that detects light, and a logic circuit that converts the light into electrical signals and data. In order to increase photosensitivity, there are ongoing efforts to increase the “fill factor” (i.e., the ratio of area of the photo sensing part to the total area of an image sensor device). However, since the logic circuit part is indispensable, there is a limit to what these efforts can achieve. Therefore, for the purpose of increasing photosensitivity, a condenser lens technique was proposed, which controls the paths of the lights incident upon nearby areas outside the photo sensing part. An image sensor using this technique has a microlens formed on the CFA.
FIG. 1A is a circuit diagram showing a conventional CMOS image sensor whose unit pixel consists of 4 MOS transistors and 1 photodiode (PD). The CMOS image sensor is provided with a photodiode 100 for receiving light and for generating photo-generated charges, a transfer transistor 101 for transferring the photo-generated charges collected by the photodiode 100 to the floating diffusion region 102, a reset transistor 103 for setting the potential of the floating diffusion region to a desired value and for resetting the floating diffusion region 102 by outputting charges, a drive transistor 104 for operating as a source follower buffer amplifier, and a select transistor for 105 for providing addressing by switching. Outside the unit pixel, there's a load transistor 106 for reading output signals.
FIG. 1B through FIG. 1G are cross-sectional views showing the manufacturing process of forming these unit pixels, for transfer transistors and reset transistors. First, a lightly doped p-type epitaxial layer 11 is formed on a heavily doped p-type substrate 10 as shown on FIG. 1B. This combination is used because the lightly doped epitaxial layer 11 improves the performance characteristics by increasing the depth of the depletion layer, and the heavily doped substrate 10 prevents crosstalk between unit pixels.
Next, a field oxide layer 12 defining active area and field area is formed on desired areas of the epitaxial layer using thermal oxide. Next, the gates of the transfer transistor 13a and the reset transistor 13b are formed on the active area by depositing gate oxide (not shown), gate polysilicon 13a and 13b, and tungsten silicide 14 and by patterning them. Although not shown on FIG. 1B, the gates of a drive transistor Dx and a select transistor Sx are also patterned.
Next, as shown on FIG. 1C and FIG. 1D, after a first mask 15 is formed to open the area where photodiodes will be formed, a n-type ion implant region 16 for a photodiode is formed inside the epitaxial layer 11 between the transfer transistor 13a and the field oxide 12 by a high-energy ion implantation process. Subsequently, a low-energy ion implantation process is performed to form a p-type ion implant region 17 for photodiode between the n-type ion implant region 16 and the surface of the epitaxial layer. Through this process, a low voltage buried photo diode (LVBPD) is completed.
Next, as shown on FIG. 1E and FIG. 1F, spacer insulation layer 18 is deposited on the whole structure, in order to form a spacer 18 made of nitride or oxide film on the sidewalls of the gate electrode of the transistors. The spacer 18 is formed on both sidewalls of the gate electrode as shown on FIG. 1F, by performing blanket dry etching process.
At this time, the surface of the photodiode can get damaged during the blanket dry etching process, causing defects in the crystal lattice structure. These defects become the source of “dark current”, or current that is caused by electrons moving from photo diode to floating diffusion region, even when there's no light present. This dark current is reported to be caused by various defects (line defect, point defect, etc.) or dangling bonds, existing near the edge of the active area. The dark current can be a serious problem in low illumination environment.
Next, as shown on FIG. 1G, a second mask 19 for forming floating diffusion region 20 and source/drain region 21 is formed, and n-type ion implantation process is performed. Next, usual subsequent processes are performed to finish the unit pixel manufacturing process.
According to the above described prior art, there is the problem that during the blanket etching for spacer formation, the surface of the photodiode becomes damaged, and dangling bonds existing on the damaged surface could cause dark current.
Also, the light incident upon the photodiode passes through the insulation film (mostly oxides) into the epitaxial layer. When the light passes from a low reflection coefficient material like the oxide to a high reflection coefficient material like the epitaxial layer, there is the problem that short wavelength lights like blue is reflected away, so photosensitivity becomes low.